1. Field of the Invention
The invention relates to a DC high voltage generator comprising an AC power supply, an AC-DC rectifier, as well as means for stabilizing the DC high voltage generator against load variations Such DC high voltage generators are well known and capable of producing DC voltages ranging from a few kV up to several MV. In particular, but not limited thereto, such DC high voltage generators are used to operate linear accelerators in which ions, electrons or other charged particles are accelerated to high energies. Beside the need for a DC high voltage, some applications of linear accelerators like electron irradiation, ion implantation and positron emission tomography (PET), require the availability of high output powers, ranging from a few hundred Watts up to several tens of kW and above. All these applications are similar in that the power from the DC high voltage generator is used to accelerate a charged particle beam originating from a suitable source.
2. Background Art
DC high voltage generators usually apply an all solid state high frequency (typically 20-200 kHz) switched mode power (SMP) converter that supplies AC power to an AC-DC rectifier comprising one or more cascade rectifiers which in turn creates the DC high output voltage. An interface between the converter and the AC-DC rectifier generally comprises a transformer, a coil and possibly additional passive electrical components in order to match the converter's impedance to that of the cascade rectifier.
In some applications, the AC-DC rectifier(s) is extended with an electrical resonant circuit to form a high voltage stack. Examples of DC high voltage generators that apply such a resonant circuit are the "Dynamitron" (see e.g. IEEE Trans. Nucl Sci. NS-16 (3)(1969),124), the "Cascade transformer high voltage generator" (U.S. Pat. No. 3,596,167), the "Nested high voltage generator" (U.S. Pat. No. 5,124,658) and a Cockcroft-Walton high voltage power supply (see e.g. IEEE Trans. Nucl Sci NS-16 (3)(1969),117).
However, the sources referred to above are unavoidably susceptible to sudden discharges in which case the charged particle beam disappears instantaneously and the needed output power is promptly reduced to nearly zero. Consequently, these applications, and others that are similar, require is an optimal transient behaviour of the DC high voltage generator.
It is well known to those skilled in the art that the output power of such DC high voltage generators is determined by the duty cycle of the switching devices in the converter, regardless of the application of one of the described resonant circuits. During variations in load, the output voltage of the generator is kept constant by regulating the duty cycle.
A drawback of such an output voltage control is that transient behavior depends on the performance of the feedback-loop and consequently overshoot and/or undershoot during transients are fundamentally unavoidable.
Another drawback of the known high frequency, high power DC generators is that switching losses present in the converter may become unacceptable if no remedial measures are taken. One possible way to eliminate these switching losses is to operate the converter in zero voltage switching mode (ZVS). ZVS is characterized in that the turn-on and turn-off of the switching devices is done at moments at which the voltage across the corresponding switching devices is close to zero. However, ZVS requires an inductive load to be present at the converter's output.
The performance of DC high voltage generators would therefore greatly benefit from an electrical design which inherently stabilizes the DC output voltage for optimal transient behavior and which furthermore enables the switching power converter to be operated in zero voltage switching mode to virtually eliminate switching losses.